The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.
Tunable lasers provide benefits to spectroscopy, atomic absorption based sensors, medical sensors and imaging and gas sensing. In these types of applications it is desirable to either sweep through a range of wavelengths in order to detect an absorption or fluorescence feature, or to tune very precisely to a particular wavelength. For instance, the absorption characteristics of different kinds of hemoglobin and water is used to sense the oxygenation of blood, as well as image the functioning of the brain or characteristics of tumors. Currently such systems used a limited number of discrete wavelengths in the wavelength range from 650-1000 nm to sample the absorption spectrum over that range. However, a device (or devices) that allow tuning over the full range would provide much more detailed information about the absorption spectrum. For sensors such as an atomic clock, the wavelength required for absorption in the atom (typically cesium or rubidium) must be much more precise than one can typically fabricate or screen for. Some tuning is required. A limited amount of tuning can be achieved by changing the current (which also results in a power change) or temperature, but practically the tuning range is limited to a couple of nanometers. A tunable device could result in a much better yield of lasers designed for this application.
Vertical cavity surface emitting lasers (VCSELs) have been widely used in short transmission distance optical networks, optical interconnects and optically based computer mice. They are now beginning to be applied more widely to consumer electronics, sensors and imaging. For miniaturized, portable, battery-driven or wearable devices, VCSELs offer the benefits of low power consumption, and very compact packaging. Other performance advantages include a very narrow linewidth, narrow beam divergence, and high modulation speed. However, other than the current or temperature tuning mentioned above, tunable VCSELs are not available. Therefore an adaptation of the conventional VCSEL structure that allows for tuning over 10's of nanometers would be advantageous for a variety applications.
Another issue in the application of VCSELs is the ability to package them effectively and combine them with lenses in a manner that allows for the efficient alignment of the VCSELs and the lenses. This is particularly important for the case where there are arrays of VCSELs combined with arrays of microlenses. Therefore, a wafer scale packaging approach that allows for the global alignment of a wafer full of lenses with a wafer of VCSEL packages would be beneficial.
One of the advantages of VCSELs over other types of semiconductor lasers is the ease with which multiple VCSELs can be integrated onto the same chip. However, the size of a given chip is limited by the ability to produce an array with good yield and uniformity. Some applications of interest in the application of VCSEL arrays include scanning across a large area. A long array of VCSELs can be used to perform the scanning without moving parts. However, the challenge in developing such a scanner is the ability to package multiple VCSEL array chips together in such a way that it creates a long array of VCSELs with a uniform pitch.